1. Field of the Invention
The invention relates to an improved method for degassing a hydraulic vehicle brake system of the type an external-force service brake system and a muscle-powered auxiliary brake system.
2. Description of the Prior Art
One vehicle brake system of the type with which this invention is concerned is known from International Patent Disclosure WO 98/28174. The external-force service brake system of the known vehicle brake system has a hydraulic pump, which can be driven by an electric motor and whose intake side is connected to a brake fluid reservoir. A hydraulic reservoir is connected to a pressure side of the hydraulic pump. The electric motor, hydraulic pump and hydraulic reservoir form an external energy source for actuating the external-force service brake system. Wheel brake cylinders are connected to the pressure side of the hydraulic pump and the hydraulic reservoir, that is, to the external energy source, and each wheel brake cylinder has a brake pressure buildup valve, which is connected between the external energy source and the wheel brake cylinder. Via a respective brake pressure reduction valve, the wheel brake cylinders are connected to the brake fluid reservoir. The brake pressure buildup valves and the brake pressure reduction valves form brake pressure control valve assemblies, with which a wheel brake pressure in the wheel brake cylinders can be set individually for each wheel. Anti-lock braking, traction control and/or vehicle dynamics control can also be performed with the brake pressure control valve assemblies. This is known per se and will not be described in further detail here.
In the event of failure of the service brake system, the known vehicle brake system has a muscle-powered auxiliary brake system. It includes a master cylinder that can be actuated by muscle power. At least one of the wheel brake cylinders of the vehicle brake system is connected to the master cylinder, with the interposition of a disconnection valve. If the service brake system fails, the disconnection valves are open or opened and the brake pressure buildup valves and brake pressure reduction valves are closed. As a result, the external energy source is disconnected hydraulically from the muscle-powered auxiliary brake system, and the wheel brake cylinders connected to the master cylinder are made to communicate hydraulically with the master cylinder, so that by actuation of the master cylinder, braking can be done with the wheel brake cylinders connected to it. During a service braking operation, the disconnection valves are closed, so that the master cylinder is disconnected hydraulically from the external-force service brake system and is not subjected to pressure by it. The service braking operation has been described above.
In external-force service brake systems that have a hydraulic pump and a hydraulic reservoir as their external energy source, the possibility exists that gas bubbles will occur in the brake fluid, causing an unwanted compressibility of the brake fluid. For instance, if the hydraulic reservoir is embodied as a gas pressure reservoir with a diaphragm that divides a reservoir gas from the brake fluid, then diffusion of reservoir gas through the diaphragm into the brake fluid is unavoidable. It is true that the gas is initially dissolved in the brake fluid, but if the quantity of gas that has diffused into the brake fluid increases and can no longer be dissolved completely, it will go out of solution and bubble out or in other words form gas bubbles. In the event of a drop in pressure of the brake fluid as well, dissolved gas can come out of solution and bubble out or form gas bubbles. The pressure drop need not at all be caused by a defect of the vehicle brake system; even typical pressure fluctuations in the hydraulic fluid of the vehicle brake system during operation can cause gas dissolved in the brake fluid to bubble out. In the case of a hydraulic reservoir that has a piston as well, it must be expected that gas will get past the piston and into the brake fluid. The hydraulic pump can also aspirate air and incorporate it into the brake fluid.
To avoid gas bubbles in the muscle-powered auxiliary brake system and in the wheel brake cylinders, cylinder and piston assemblies are provided, in the vehicle brake system of the aforementioned WO 98/28174, for those wheel brake cylinders that are connected to the master cylinder. The cylinder-piston assemblies form media separators, which separate the brake fluid of the external-force service brake system from the brake fluid of the muscle-powered auxiliary brake system and of the wheel brake cylinders. This prevents gas from the brake fluid in the external-force service brake system from getting into the brake fluid of the muscle-powered auxiliary brake system and into the wheel brake cylinders. Compressibility of the brake fluid from gas bubbles in the muscle-powered auxiliary brake system is avoided. Compressibility of the brake fluid of the muscle-powered auxiliary brake system would lessen its braking action or put the auxiliary brake system out of operation. Gas bubbles in the brake fluid in the external-force service brake system and an attendant compressibility of the brake fluid are tolerated as a compromise in the known vehicle brake system, since the hydraulic pump and the hydraulic reservoir make an adequate quantity of brake fluid available so that despite brake fluid compressibility, an adequate brake fluid pressure can be built up in the wheel brake cylinders.